(1) Field of the Invention
The present invention relates to a resin composition for formation of a closed-cell foam of a thermosetting polyester resin having a uniform cell size and a uniform cell distribution. Furthermore, the present invention relates to a cured resin foam excellent in the heat resistance, which is prepared from this resin composition.
(2) Description of the Prior Art
A setting polymerizable composition comprising an unsaturated polyester prepolymer and an ethylenically unsaturated monomer is known and called "an unsaturated polyester resin". By utilizing excellent mechanical properties and high chemical resistance, this composition is widely used for formation of various structures and liners of various devices.
It has also been known from old that this thermosetting polyester resin is formed into a foamed structure. For example, there are known a process in which a volatile blowing agent and a nucleating agent are incorporated into a setting polymerizable composition and blowing is effected (see the specification of U.S. Pat. No. 3,884,844), a process in which blowing is effected by using a bicarbonate (see the specification of U.S. Pat. No. 4,028,289) and a process in which a carbonate and/or bicarbonate and an aqueous solution of basic aluminum chloride are incorporated in a setting polymerizable composition and blowing is effected (see the specification of U.S. Pat. No. 4,347,331).
In the production of a foam of a thermosetting polyester resin, it is difficult to uniformly disperse a blowing agent or nucleating agent in a setting resin composition, and if the blowing agent is not uniformly dispersed, the state of dispersion of cells becomes heterogeneous and the cell size becomes uneven. Moreover, it is difficult to cause blowing of the composition, thickening of the composition and curing polymerization at a good timing. For example, if blowing and thickening are not performed at a good timing, breakage or collapse of cells is caused or cells are connected to one another, resulting in reduction of the physical properties and the expansion ratio of the obtained foam. Furthermore, if blowing is caused in the stage where curing polymerization is being advanced, cell walls of the foam are broken or torn or a stress is left in the cell walls, and also in this case, the physical properties of the foam are reduced.
Still further, in view of the properties of the obtained cured resin foam, it is difficult to obtain a product in which cells are uniform through the entire structure and the expansion ratio is high, and even if a foamed product having a high expansion ratio is obtained, this product is defective in that many cracks are formed in the interior by the heat generated at the time of curing.